1. Field of the Invention
The present invention relates to an apparatus for preventing a grinding wheel from rotating at an excessive circumferential speed, more than a predetermined speed, due to an improper selective use of motor pulleys.
2. Description of the Prior Art
In a grinding technology, generally a motor pulley on a motor shaft is exchanged with a reserved motor pulley larger in diameter than the first mentioned motor pulley, thereby to recover a circumferential speed of a grinding wheel when the same wears by a predetermined amount. However, unless the reserved motor pulley is exchanged with the previously used pulley having a small diameter when the grinding wheel wears beyond a useable or tolerable range to be replaced by a fresh grinding wheel, this fresh wheel is rotated at an excessive speed, more than a predetermined speed, and gets into danger of destruction.
In order to avoid such danger, there have been developed wheel circumferential overspeeding preventing apparatus of the kind that only a small diameter motor pulley is attachable on an electric motor while a large diameter grinding wheel is being mounted on a wheel spindle of a wheel head, and such apparatus provides on the wheel head both a rejection member for detecting the diameter of the grinding wheel and another rejection member for restraining a large pulley from attachment while the diameter of the grinding wheel is large.
Because of such construction, the distance between the motor pulley and the rejection member opposing the same must be further extended when, for the purpose of tension adjustment or compensation for belt stretch, the position of the motor base is adjusted to be further distanced from the grinding wheel so as to thereby lengthen the distance between the motor pulley and the wheel spindle. Consequently, although the grinding wheel is not yet reduced to a diameter for pulley exchange, a large diameter pulley is put in a condition to be attachable to the motor shaft, so that the grinding wheel may be in danger of being rotated at an excessive circumferential speed.
In the case where the support position of the other rejection member is decided, taking belt stretch into consideration, the foregoing drawback can be solved. In this case, however, while the belts remain almost fresh or unstretched, an exchange of the motor pulley to the large one is impossible, even when the grinding wheel becomes small beyond a predetermined diameter to decrease the circumferential speed of the grinding wheel below a tolerable speed, and, therefore, there is raised another drawback that the wheel circumferential speed cannot be maintained within a tolerable range.